1. Field of the Invention
The present invention relates to an improvement of a back layer of a thermosensitive recording material having a support made of a polypropylene film, in other words, to a thermosensitive recording material which is excellent in binding property, antistatic property, and adhesion of water-resistant adhesive, and which is free from the blocking to the surface of the thermosensitive recording material in a wound-up state.
2. Description of the Related Art
Recently, the diversification of information and enlargement of needs have urged the study and development of various recording materials in the field of information recording, and various materials have been put to practical use. Among them, a thermosensitive recording material is advantageous in that:
(1) it allows easy recording of images with mere heating process,
(2) a device therefor can have a simple mechanism and can easily be downsized; and
(3) it is easy to handle and inexpensive.
Accordingly, a thermosensitive recording material is widely utilized in various fields, including data processing (the output of a desk-top calculator, computer, or the like), a recorder for a medical measurement, a low-speed or high-speed facsimile, a ticket vending machine (railway ticket, admission ticket, or the like), a thermal copying, a label for a POS system, a tab, and so on.
Typically, the thermosensitive recording material has, on a paper support, a layer containing a color former which generates a coloring reaction by heat. However, a synthetic paper or synthetic resin film is used as a support in view of dimensional stability, physical strength, and insolubility to water, which are properties required for a recording paper. However, since such a film support as synthetic paper support has high electric resistance compared to a paper support, it tends to generate static electricity by the friction with a platen roll, a thermal head, or the like when the recording material is conveyed for the printing in a printer, followed by a running fault such as jamming, a damage of the thermal head, or the like.
Since such static electricity is mainly generated by the friction between a platen roll and a recording material, an antistatic agent has commonly been applied onto the back side of a thermosensitive recording material, the surface to be contacted with platen rolls. Examples of the antistatic agents used include (1) inorganic salts such as sodium chloride, (2) anionic polymer electrolytes such as sodium polystyrene sulfonate, and (3) conductive metallic compounds such as conductive zinc oxide and tin oxide.
However, inorganic salts (1) and anionic polymer electrolytes (2) are less effective for the amount used, and are sticky under a high humidity condition and easy to dissolve into water because of their nature as electrolytes. Conductive metallic compounds (3) are not affected by humidity and are capable of displaying a desired effect even with a slight amount; however, they have a serious drawback in chemical safety as a thermosensitive recording material.
Meanwhile, especially in the field of POS label, tag, and CAD, there is a demand for a thermosensitive recording material satisfying concurrently properties such as physical strength against bending or breaking, dimension stability, and water-insolubility so as to meet requirements in the field.
In view of such requirements, Japanese Patent Application Laid-Open (JP-A) No. 06-234270 discloses a thermosensitive recording material having, on the back surface, an antistatic layer containing as main components a quaternary salt polymer, a water-soluble resin, and a water-resistance imparting agent. The thermosensitive recording material is described as having an antistatic property and water resistance, reducing occurrence of running failure and occurrence of a thermal head damage, showing no sticking under a high humidity condition, and having a high chemical safety.
However, this thermosensitive recording material has drawbacks that the adhesion between the antistatic layer, the support and an adhesive layer is so weak that the antistatic layer easily peels off from the support. It also has a drawback that, after dipping in water, the antistatic layer or the adhesive layer is easily peeled off from the support only by rubbing it with a finger. This is a serious problem in the case where an adhesive layer and a release paper are successively laminated.
In addition, the back surface easily adheres to a surface while the recording material is stored in the form of a roll after a coating process, so that the coated layer disposed either on the front or back surface tends to peel off after the recording material is unwound.
JP-A No. 2002-248864 discloses a thermosensitive recording material having, between the support and the heat developing layer and/or on the back surface of the support, an antistatic layer containing a quaternary salt polymer and an acryl-composite urethane resin, in order to obtain a thermosensitive recording material excellent in a film strength, running performance, and water resistance.
JP-A No. 2005-81626 discloses a thermosensitive recording material having a back layer containing a non-foaming plastic hollow filler, a styrene-acryl copolymer having an acid value of 200 or more and a glass transition temperature (hereinafter referred to as “Tg”) of 80° C. or more, and an acrylic resin emulsion on the back surface of the support made of plastic film or synthetic paper, in order to obtain a thermosensitive recording material which is excellent in the anti-blocking in the storage as a roll, in antistatic property, and in adhesion property between the support and the layers, in which the adhesive does not peel off from the back surface even when water penetrates into the adhering surface.
JP-A No. 2004-98634 discloses a thermosensitive recording material containing an adhesive including a polyurethane resin adhesive and one of an acrylic resin adhesive having a Tg of 30° C. to 220° C. and an acrylamide resin adhesive wherein the amount of the polyurethane resin adhesive is 3% by mass to 30% by mass with respect to the whole amount of the adhesives, in order to obtain a thermosensitive recording material which causes no blocking when superposed on any surface and has excellent binding property.
JP-A No. 2006-289962 discloses a thermosensitive recording material excellent in antistatic property and binding property, which contains a copolymer of (meth)acryloyloxyalkylammonium salt and styrene in any of the coated layers, or which contains a copolymer of (meth)acryloyloxyalkylammonium salt and styrene and as an adhesive resin acrylic acid ester copolymer in the back layer.
In recent years, in order to reduce the cost for producing a plastic film and to improve glossiness of a film, a plastic film has been developed in which one side or both sides of the support is extremely smooth and glossy. However, the technique disclosed in any of the above-mentioned patent literature is not applicable for all types of plastic film support, and shows little or no effect when using a support having extremely smooth surface (for example, a smoothness of 5,000 seconds or more measured by Oken type tester), causing a problem that binding property of the layer with the support is so weak that the layers easily peels off and that an adhesive dissolves in water and peels off from the support.